Imaging devices based on digital image sensors operate by capturing an optical image and converting it into electronic signals. Such devices are capable of capturing still images and/or a video composed of a sequence of images, often referred to as frames. A digital image sensor typically includes photosensitive cells (often referred to as photosites) arranged in an array of rows and columns. Photosensitive cells used in imaging devices are often based on semiconductor technology, capable of converting photons into electrons, such as, for example, charge coupled devices (CCD) or complementary metal oxide technology (CMOS). Such imaging sensor devices have wide ranging applications ranging from professional film studio cameras to consumer devices including dedicated imaging devices such as camcorder cameras as well as personal electronic devices with built in imaging functions, such as mobile telephones, smart telephones, personal digital assistants (PDA), and portable computers including tablet computers, laptop computers, notebook computers, and the like.
Imaging devices based on CCD technology typically employ a global shutter technique in which an entire array of photosensitive cells is exposed at the same time to capture a frame of image data. Imagers based on CMOS technology, on the other hand, typically employ a “rolling shutter” technique to capture a frame. In this so called “rolling shutter” technique the scan lines (for example, rows or columns) of the sensor array are each exposed at different times, and read out from the photosensitive cells is performed sequentially line by line (for example, from the top row of the sensor array to the bottom row of the sensor array). The read outs from the lines of photosensitive cells are then merged together to form a single image.
When an object to be imaged is fixed with respect to the imaging device, or is slow moving with respect to the processing time of all the lines, the rolling shutter has no significant effect on image quality. If the lighting conditions are constant with respect to the processing time, again the rolling shutter has no significant effect on image quality. However, in scenarios where an object is fast moving and/or the lighting conditions change during acquisition of the entire image, the effects can be detrimental to the quality of the images obtained.
In image capture, a flash is often used to improve image quality by illuminating the scene with a burst or pulse of visible light while a frame of the scene is being captured. In an imaging device equipped with a rolling shutter, however, the flash can have adverse effects on the image quality due to the line by line processing technique. Indeed, since the duration of a flash may be short (typically of the order of tens to hundreds of μs) compared to the processing time of a frame (typically of the order of tens of ms) the different lines of sensors will be exposed in significantly different ways. This difference in exposures leads to effects such as partial lighting which impair the quality of the images obtained and to image distortions such as skew type effects. The impairment of image quality may be further aggravated in the case of the frame capture of fast moving objects leading to image artifacts such as skew effects.
The present invention has been devised with the foregoing in mind.